A power supply system including an element connected to a remote power supply source or power consumer through an electric power supply device. The element is movably arranged in relation the source or consumer in an environment in which at least a part of the power supply device is subjected to water and water vapor and to repeated bending motion due to the movement of the element. The electric power supply device includes an electric cable that includes a conductor and at least one layer of an electrically insulating material applied on the outside of the conductor. The device includes a member for preventing water and vapor intrusion into the layer. The member includes a flexible, watertight, vapor impermeable, fatigue resistant, separate tubing that surrounds the electric cable.